Bovine respiratory disease is associated with a large number of pathogens, including viruses (BVD, IBR, BRSV, Coronavirus, and many others) and bacteria (Mannheimia haemolytica [formerly Pasteurella haemolytica], Pasteurella multocida, Haemophilus somnus, Mycoplasma sp, and others). These pathogens nearly always gain entry to the lungs through the upper respiratory tract (nose, throat, and trachea).
Before birth, the respiratory tract is sterile, but pathogens begin to inhabit the upper respiratory tract soon after birth. If samples from several healthy calves are collected, some will yield major bacterial pathogens. Pathogens can be present and cause no illness because host defenses limit them to the upper airways. The ability to recover organisms from apparently healthy cattle is increased by "stressing" them - moving to a new location, commingling with others, withholding water, changing feed, etc.
When calf defenses cannot contain pathogens to the upper airways, respiratory disease will result. Calves with pneumonia act as "incubators", expelling pathogens in respiratory mucus. Presence of a sick calf forces healthy penmates to increase their host defense to compensate for increased exposure. If the healthy calf can do this, it will remain healthy; if it cannot increase host defenses, then it will become sick too.
Why are young calves so susceptible to bovine respiratory disease (BRD) compared to yearling age and older cattle? It has a lot to do with host defense. While older cattle aren't "bulletproof", they are far less likely to be affected with BRD, unless they are commingled from multiple sources and stressed.
Young calves have a less capable defense system. The antibodies in the colostrum they received shortly after birth decay by 50% every three weeks - so many calves may have little antibody left at weaning. Calves are less able to cope with adversity. They are not accustomed to other animals, feedbunks, and water tanks, so they may not eat well. They may not find a place to rest. They are more susceptible to the stress of change.
The control of BRD relies on control of stressors in the environment, and improvement of the calf defense response.
1. Weaning (withdrawal of milk and replacement with solid feed) is an unavoidable stress. Generally, it is not recommended to wean prior to four weeks of age. Weaning stress can be minimized by transitioning onto high quality feedstuffs over a period of time. Transition options include:
a. Decreasing milk feeding to 50% for 1 week then withdrawing milk.